The present invention relates to an optical modulator of clock modulation type.
In accordance with recent increases in data transmission rates, development of an optical modulator for modulating a data signal into an optical signal from an electric signal has been performed vigorously in the field of long-distance optical communications systems, such as in a submarine optical communication system. In the field of long distance optical communications systems such as in submarine optical communication, attention has been paid to a transmission scheme using an RZ (return-to-zero) signal as an optical signal to be transmitted rather than to another transmission scheme using an NRZ (non-return-to-zero) signal, because the transmission scheme using an RZ signal has a wide range of tolerance of dispersion and an improved receiving sensitivity characteristic.
When RZ optical modulation is effected through use of an external optical modulator in order to transmit the previously-described optical RZ signal, there is a necessity for a driver circuit which is higher in frequency than a driver circuit for effecting NRZ optical modulation. Manufacture of a driver circuit for subjecting data of 40 Gb/s to RZ modulation using a current semiconductor element is difficult.
The present invention is aimed at providing an optical modulator of clock modulation type which integrates, into a single chip, two modulators; that is, a modulator for effecting encoding through use of an NRZ electric signal and a modulator for generating an RZ signal, thereby enabling reduction in space while improving the tolerance of the optical modulator and reducing costs required for constructing the same.
To this end, the present invention provides an optical modulator of clock modulation type comprising:
a substrate possessing an electro-optical effect;
a first optical waveguide of Mach-Zehnder type formed on the substrate;
a second optical waveguide of Mach-Zehnder type formed on the substrate so as to be concatenated with the first optical waveguide of Mach-Zehnder type;
a first electrode formed on the substrate for controlling light propagating through the first optical waveguide;
a second electrode formed on the substrate for controlling light propagating through the second optical waveguide;
the first optical waveguide, the second optical waveguide, the first electrode, and the second electrode being integrated in the substrate;
a clock signal generator which is connected to either the first or second electrode and which produces an RZ (return-to-zero) signal by applying a clock signal to either the first or second electrode; and
an NRZ (non-return-to-zero) data signal generator which is connected to either the first or second electrode and supplies an NRZ data signal to either the first or second electrode.
Thus, in the optical modulator of clock modulation type according to the present invention, the first optical waveguide, the second optical waveguide, the first electrode, and the second electrode are integrated in the substrate. The optical modulator is further provided with the clock signal generator and the NRZ data signal generator. Thus, the space required by the optical modulator is reduced while tolerance of the same is improved, thus diminishing costs required for constructing the optical modulator.
Preferably, the first optical waveguide, the second optical waveguide, the first electrode, and the second electrode may be integrated in the substrate in a single chip. Further, each of the first and second electrodes may preferably be constituted of a signal electrode and a ground electrode, and the ground electrode may preferably be shared between the first and second electrodes.
Further, in a preferable configuration, one of the first or second electrodes is constituted as a dual electrode having two signal electrodes, and the remaining electrode is constituted as a single electrode having one signal electrode.
Alternatively, each of the first and second electrodes maybe constituted as a dual electrode having two signal electrodes, or each of the first and second electrodes may be constituted as a single electrode having one signal electrode.
Preferably, the clock signal generator may be constituted so as to generate an RZ signal having a transmission speed equal to a per-unit-time transmission speed of light output from the optical modulator of clock modulation type, by applying, to either the first or second electrode, a clock signal having a frequency which corresponds to half the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type.
In this case, the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type may preferably be set to a value of at least 10 Gb/s or more, and the frequency of the clock signal may preferably be set to a value of 5 GHz or more. Preferably, the transmission speed may be set to a value of 40 Gb/s or more, and the frequency may be set to a value of 20 GHz or more.
In the present invention, the optical modulator is provided with the clock signal generator which applies, to either the first or second electrode, a clock signal having a frequency which is half the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type. As a result, a clock signal generator is less costly to constitute than a clock signal generator which applies, to either the first or second electrode, a clock signal having a frequency corresponding to the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type.
Moreover, the clock signal generator may preferably be constituted so as to generate an RZ signal having a transmission speed equal to a per-unit-time transmission speed of light output from the optical modulator of clock modulation type, by applying, to either the first or second electrode, a clock signal having a frequency corresponding to the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type.
In this case, the per-unit-time transmission speed of the light output from the optical modulator of clock modulation type may preferably be set to a value of at least 10 Gb/s or more, or, more preferably, a value of 40 Gb/s or more, and the frequency of the clock signal may preferably be set to a value of 10 GHz or more, or, more preferably, a value of 40 GHz or more.
Preferably, the substrate may be formed from lithium niobate, lithium tantalate, or a lithium niobate crystal, and the substrate may be cut along a Z axis.
According to the present invention, since the substrate is cut along the Z-axis, matching can be readily achieved in connection with parameters to be used for evaluating performance when a high-speed optical modulator is constructed in connection with four parameters; namely, a drive voltage, a match between the speed of a light signal and the speed of an electric signal, an attenuation constant of an electric signal, and a characteristic impedance.
Further, the first and second optical waveguides are concatenated with each other by means of a concatenate waveguide, and light loss reduction means can be formed in the vicinity of the concatenate waveguide.
The present invention also provides an optical modulator of clock modulation type comprising:
a substrate formed from lithium niobate;
a first optical waveguide of Mach-Zehnder type formed on the substrate;
a second optical waveguide of Mach-Zehnder type formed on the substrate so as to be connected with the first optical waveguide of Mach-Zehnder type;
a first electrode formed on the substrate for controlling light propagating through the first optical waveguide;
a second electrode formed on the substrate for controlling light propagating through the second optical waveguide;
the first optical waveguide, the second optical waveguide, the first electrode, and the second electrode being integrated in the substrate;
a clock signal generator which is connected to the first electrode and produces an RZ signal by applying a clock signal to the first electrode; and
an NRZ data signal generator which is connected to the second electrode and supplies an NRZ data signal to the second electrode.
Thus, in the optical modulator of clock modulation type according to the present invention, the first optical waveguide, the second optical waveguide, the first electrode, and the second electrode are integrated in one chip on the substrate. The optical modulator is further provided with the clock signal generator and the NRZ data signal generator. Thus, the space required by the optical modulator is reduced while tolerance of the same is improved, thus reducing costs required for constructing the optical modulator.
Preferably, each of the first and second electrodes may be constituted of a signal electrode and a ground electrode, and a ground electrode may be shared between the first and second electrodes.
Furthermore, either the first electrode or the second electrode may preferably be constituted as a dual electrode having two signal electrodes, and the remaining one of the first and second electrodes may preferably be constituted as a single electrode having one signal electrode. Moreover, each of the first electrode and the second electrode may preferably be constituted as a dual electrode having two signal electrodes, or each of the first electrode and the second electrode may preferably be constituted as a single electrode having one signal electrode.
Preferably, the substrate may be cut along a Z axis.
According to the present invention, since the substrate is cut along the Z-axis, matching can be readily achieved in connection with parameters to be used for evaluating performance when a high-speed optical modulator is constructed in connection with three parameters; namely, a drive voltage, a match between the speed of a light signal and the speed of an electric signal, an attenuation constant of an electric signal, and a characteristic impedance.
The optical modulator of clock modulation type according to the present invention is constituted of a first Mach-Zehnder optical modulator, a second Mach-Zehnder optical modulator, a clock signal generator, and an NRZ data signal generator.
More specifically, the first Mach-Zehnder optical modulator includes a substrate possessing an electro-optical effect; a first optical waveguide of Mach-Zehnder type formed on the substrate; and a first electrode formed on the substrate for controlling light propagating through the first optical waveguide.
Further, the second Mach-Zehnder optical modulator is concatenated with the first Mach-Zehnder optical modulator, includes the substrate shared with the first Mach-Zehnder optical modulator, a second optical waveguide of Mach-Zehnder type formed on the substrate and connected to the first optical waveguide and a second electrode formed on the substrate for controlling light propagating through the second optical waveguide, and is integrated with the first Mach-Zehnder optical modulator.
Moreover, the clock signal generator is connected to either the first or second electrode and produces an RZ signal by applying a clock signal to the same.
The NRZ data signal generator is connected to the remaining one of the first and second electrodes and supplies an NRZ data signal to the same.
Consequently, the optical modulator of clock modulation type according to the present invention is constituted by integrating a modulator for effecting encoding operation using an NRZ electric signal, and a modulator for generating an RZ signal. As a result, the space required by an optical modulator can be reduced while the tolerance of the optical modulator is improved. Thus, the optical modulator has advantageously lower construction costs.